The present invention relates to a radio frequency device; and, more particularly, to a radio frequency device using a micro-electronic-mechanical system (MEMS) technology.
Generally, a micro-electronic-mechanical system (MEMS) technology is called a micromachining, micro-system or ultra-small size precise machine technology. The technology is used to manufacture ultra-small three-dimensional structure by processing a wafer.
The methods for applying the MEMS technology to the radio frequency (RF) area are studied actively, especially in the areas of radio communication and national security. In particular, the low-loss RF switch and low-loss filter draw explosive attention from the radio communication area.
The low-loss RF switch uses an electrostatic attractive force. The switch has two types: one moving the beams of the switch right and left, and the other moving them up and down. The two types of low-loss RF switches are divided again into a direct contact switch (or it is called a resistive switch) and a capacitive switch.
The conventional resistive or capacitive MEMS switch is mounted on a substrate. A top electrode is formed in the form of a cantilever or a membrane, and it works as an actuator, which makes a movement by the electrostatic attractive force with a bottom electrode, which is a signal line. The conventional resistive or capacitive MEMS switch uses the principle of the top electrode and the bottom electrode connected to each other through the electrostatic attractive force to transmit an RF signal.
In case where the resistive MEMS switch is desired to be operated under an operating voltage of 3V in the current mobile communication area, the spring constant k should be as sufficiently small as 1 N/mxcx9c3 N/m. To make the spring constant that small, the physical length of the switch should be longer than 500 xcexcm. After all, this increase in the physical length drops the reliability of the MEMS switch device, and increases the switching time as much as several milliseconds.
Meanwhile, if the physical length of the MEMS switch device is reduced, a problem of increasing operating voltage emerges. Therefore, researchers are studying to develop a switch with short physical length and small spring constant.
In case where a capacitive MEMS switch should be operated at a high speed of several microseconds (xcexcs), more than 20V of high operating voltage is required. To speed up the switch, various efforts have been attempted, such as making an air hole in the actuator to thereby reduce the mass, or modifying the shape of the actuator to make the spring constant small and thus reduce the operating voltage and improve the switch rate of the switch.
As described above, low operating voltage and rapid switching time are required to apply the switch, which can be operated in the RF range, to the mobile communication terminal.
In case of the capacitive MEMS switch, operating voltage as high as 50V should be supplied to make the switch operate at a high speed of 4-6 xcexcs. [Z. Jamie Yao, Shea Chen, Susan Eshelman, David Denniston and Chuck xe2x80x9cMicromachined Low-Loss Microwave Switches,xe2x80x9d IEEE Journal of Micro-electro-mechanical Systems, Vol. 8, pp. 129, 1999]
Meanwhile, when the capacitive switch that operates at a high voltage is embodied to operate at a low temperature, the operation of the switch needs to be optimized according to the shape change of a bridge structure, and the air gap has to be smaller. However, when the air gap is reduced, the isolation of the RF signal is deteriorated. Therefore, the air gap should be maintained around 1-4 xcexcm. [J. M. Huang, K. M. Liew, C. H. Wong, S. Rajendran, M. J. Tan and A. Q. Liu, xe2x80x9cMechanical Design and Optimization of Capacitive Micromachined Switch,xe2x80x9d Sensors and Actuators A 93 pp. 273, 2001]
Particularly, since the switching characteristic of the capacitive MEMS switch is more improved, as the capacitance ratio between on and off is large, a dielectric substance having a higher dielectric rate may be applied. [G. M. Rebeiz and J. B. Muldavin, xe2x80x9cRF MEMS Switches and Switch Circuit,xe2x80x9d IEEE Microwave Magazine, Vol. 2, pp. 67, 2001; and Wallace W. Martin, Yu-Pei Chen, Byron Williams, Jose Melendez and Darius L. Crenshaw, xe2x80x9cMicro-electronic-mechanical Switch with Fixed Metal Electrode Dielectric Interface with a Protective Cap Layer,xe2x80x9d U.S. Pat. No. 6,376,787, April, 2002.] However, the capacitive MEMS switch still operates at a high operating voltage over 20V.
When the resistive MEMS switch is embodied to be operated under 3V, which is the operating voltage in the current mobile communication area, the spring constant k should be as sufficiently small as 1xcx9c3 N/m. Accordingly, the physical length of the switch becomes as long as more than 500 xcexcm, thus causing a problem in the device reliability and switching rate. [Robert Y. Loo, Adele Schmitz, Julia Brown, Jonathan Lynch, Debabani Cohoudhury, James Foshaar, Daniel J. Hyman, Juan Lam, Tsung-Yuan Hsu, Jae Lee, Mehran Mehregany xe2x80x9cDesign and Fabrication of Broadband Surface-Micromachined micro-electro-mechanical Switches for Microwave and Millimeter Wave Applications,xe2x80x9d U.S. Pat. No. 6,046,659, April, 2000; and L. R. Sloan, C. T. Sullivan, C. P. Tigges, C. E. Sandowal, D. W. Palmer, s. Hietala, T. R. Christenson, C. W. Dyck, T. A. Plut, and G. R. Schuster xe2x80x9cRF Micro-mechanical Switches That Can Be Post Processes on Commercial MMIC,xe2x80x9d Electric Component and Technology Conference 2001.]
Meanwhile, when the physical length of the switch device is shortened, there is a problem that the operating voltage is raised. So, researchers are studying to find a MEMS switch of a new structure using an electrostatic attractive force, and a new material. When a new material is to be found, the area of the membrane should be large and the mass should be small to make the switch operate at a low voltage, and these conditions are contrary to each other.
Hereinfrom, the conventional resistive and capacitive MEMS switches are described with embodiments.
FIG. 1 is a plane figure showing a conventional membrane-type capacitive switch, and FIG. 2 is a cross-sectional view illustrating the capacitive switch of FIG. 1, cut along the line a-axe2x80x2.
Referring to FIGS. 1 and 2, a conventional capacitive switch 18, which is fabricated in the micro-fabrication process technique, such as photolithography, etching, deposition and lifting-off, is provided to a substrate 10 having such a characteristic as insulation, semi-insulation or semiconduction, and polymerization.
The capacitive switch 18 largely has two parts: a part fixed on the substrate 10 (to be referred to as a fixed part, herefrom), and the other part that makes a mechanical movement, that is, actuating part (to be referred to as an actuator, herefrom).
The part fixed on the substrate 10 includes an insulation layer 11, a bottom electrode 12, a capacitive dielectric layer 13, and a grounding surface 17, and the actuator includes a top electrode 15.
To be more concretely, the insulation layer 11 is formed on the substrate 10, and a plurality of grounding surfaces 17, which are connected with an active zone (not shown) formed inside the substrate 10 or the conduction layer, are embodied and arranged through metal wires. Between the grounding surfaces 17, there is the bottom electrodes laid, and on the bottom electrode 12, the dielectric layer 13 covering the bottom electrode 12 is positioned. On top of the dielectric layer 13, there is the top electrode 15 supported by the supporting material 14 positioned at both ends of the insulation layer 11. Therefore, the top electrode 15 forms a membrane structure having a regular space (d) with the dielectric layer 13 under the top electrode 15 by the cavity formed in the lower part of the top electrode 15.
The top electrode 15 is an actuator. S, when an electric voltage is supplied to the top electrode 15, the top electrode 15 is drawn to the bottom electrode 12 by the electrostatic attractive force generated by its potential difference with the bottom electrode 12 and contacts the dielectric layer 13.
Here, since the top electrode 15 and the bottom electrode 12 are formed of a metal, such as Al and Cu, the top electrode 15, dielectric layer 13 and the bottom electrode 12 form a MIM capacitor having a metal electrode, in which a dielectric substance is between the metals. Accordingly, an external RF signal supplied through the bottom electrode 12 is shut by the capacitor, and the grounding surface 17 grounds the RF and direct current (DC).
Referring to FIG. 2, when the top electrode 15 and the bottom electrode 12 are separated by an air layer having a space (d), the RF signal is transmitted to the bottom electrode 12. Here, the larger the dielectric constant of the dielectric layer 13 is, the bigger the capacity is and the better the shutting characteristic becomes.
However, when the space (d) becomes narrower, the RF signal isolation of the switch 18 is degraded and the process of making the space (d) narrower has a technical limitation, too.
FIG. 3 is a plane figure showing a conventional membrane-type resistive switch, and FIG. 4 is a cross-sectional view illustrating the resistive switch of FIG. 3, cut along the line b-bxe2x80x2.
Referring to FIGS. 3 and 4, a resistive switch 28 includes a bottom electrode 21 and a supporting material 22 fixed on the substrate 20, and an contact pad 23, which is an actuator, an insulation membrane 24, and a top electrode 26.
To be more concrete, a plurality of bottom electrodes 21 are arrayed on the substrate 20, and on top of the bottom electrode 21, the membrane 24 is positioned by the supporting material 22 at both ends of the substrate.
Here, the membrane 24 is formed of such a material as nitride layer having a conventional compressibility and extensibility. The membrane 24 has a regular space with the bottom electrode 21 under the membrane 24 by the cavity 25 formed in the lower part of the membrane 24. The contact pad 23 is positioned on one surface of the membrane 24 that confronts the bottom electrode 21. So, the membrane 24 is drawn toward the bottom electrode 21 by the electrostatic attractive force between the top electrode 26 and the bottom electrode 21 and contacts the bottom electrode 21. The top electrode 26 is positioned on top of the membrane 24, that is, on a surface of the membrane 24 that does not confront the bottom electrode 21.
The bottom electrode 21 and the contact pad 23, to which the RF signal inputted via signal line 27 is inputted, are in the off state. When a DC is supplied to the top electrode 26, the membrane 24 moves towards the bottom electrode 21 by the electrostatic attractive force between the top electrode 26 and the bottom electrode 21, and thus the membrane 24 contacts the bottom electrode 21. This is the on state.
Here, if the DC supplied to the top electrode 26 is shut, the bottom electrode 21 and the contact pad 23 are separated and the state is converted back into the off state by the elastic restoring force of the membrane 24, of which both ends are fixed on the substrate 20 by the supporting material 22. In the off state as shown in FIG. 4, the contact pad 23 is separated from the bottom electrode 21. Therefore, the RF signal supplied to the bottom electrode 21 stops flowing.
Meanwhile, to embody the membrane-type resistive switch to operate at a low voltage, the spring constant k of the membrane 24 should be small. To make the spring constant k of the membrane 24 small, the physical lengths of the top electrode 26 and the membrane 24 should be long. Therefore, although the operating voltage could be low, it takes longer time for the switch to go back to the off state by the restoring force. Due to this correlation between the physical length and the operating voltage, technically, it is very hard to form a high-speed switch that operates at a low voltage.
As described above, resistive and capacitive MEMS switches should necessarily be operated at a high-speed at a low voltage in order to be applied to a mobile communication area. To be operated at a high-speed at a low voltage, they should be able to satisfy the following conditions.
A resistive switch, both membrane type and cantilever type alike, should have short physical length and small spring constant of the actuator. In case of a capacitive switch, the capacity ratio of the on and off states should be raised, and the air gap and the operating voltage should be lowered necessarily.
It is, therefore, an object of the present invention to provide a radio frequency (RF) device using a micro-electronic-mechanical system (MEMS) technology that can be applied to a mobile communication area by reducing the operating voltage while heightening the operating rate of the RF device.
In accordance with an aspect of the present invention, there is provided a radio frequency device using a micro-electronic-mechanical system (MEMS) technology, comprising: a substrate; a first electrode which is mounted on the substrate and forms an actuator, part of the first electrode not contacting the substrate; and a second electrode which is apart in a regular space from the substrate and forms an actuator, part of the second electrode being overlapped with the first electrode, wherein the first electrode and the second electrode contact each other at a contact point by an electrostatic attractive force generated between the two electrodes.
In accordance with another aspect of the present invention, there is provided a radio frequency device using a MEMS technology, comprising: a substrate; a first electrode which is mounted on the substrate, and forms an actuator, part of the first electrode not contacting the substrate; a second electrode which is apart in a regular space from the substrate and forms an actuator, part of the second electrode being overlapped with the first electrode; and a third electrode which is apart in a regular space from the circumferential surface of the substrate and forms an actuator, part of the second electrode being overlapped with the second electrode, wherein the first electrode and the second electrode contact each other at a contact point by an electrostatic attractive force generated between the first electrode and the second electrode, and an electrostatic repulsive force generated between the second electrode and the third electrode.